Here's why other possibilities are less likely and why the differential is the prime suspect:
* Not likely engine related: Engine problems usually manifest differently. A rod knock would be present at all speeds and not necessarily be louder at higher RPMs. Valvetrain issues would likely be heard at higher RPMs, but not necessarily persist when the vehicle is stopped.
* Differential/Axle Issues: The clicking sound originating from the bottom of the engine bay, starting after a certain speed, and being RPM-dependent strongly suggests a problem within the drivetrain *after* the transmission. Here are the most likely culprits:
* Worn differential bearings: These bearings support the gears inside the differential. As they wear, they create a clicking or knocking sound that worsens with speed and load (higher RPM).
* Damaged differential pinion gear: A damaged or worn pinion gear can also create a clicking noise, especially under load.
* Loose or damaged axle shaft: While less common, a loose or damaged axle shaft can also produce clicking sounds, especially during turns or under load.
* U-joint problem (though less likely given location): While U-joints are more commonly associated with a clunking sound, severe wear could manifest as clicking, particularly at higher speeds. However, the location you describe makes this less likely than the differential issues.
What to do:
1. Don't drive it much further: Continuing to drive with a damaged differential can cause catastrophic failure, potentially leading to significant damage and repair costs.
2. Get it professionally inspected: A mechanic specializing in older vehicles should inspect the differential, axles, and related components. They'll likely need to inspect it on a lift, likely including removing the differential cover for a visual and audible inspection. This is crucial for proper diagnosis.
Ignoring this problem could lead to much more expensive repairs down the line. Get it checked out as soon as possible.
